Exhaust gas driven turbochargers include a rotating shaft carrying a turbine wheel and a compressor wheel, which is rotatably supported within a center housing by one or more bearings. During operation, exhaust gas from an internal combustion engine drives a turbocharger's turbine wheel, which, in turn, drives the compressor wheel to boost charge air to the internal combustion engine.
For increased performance (e.g., power, emissions, etc.), some turbochargers include a variable geometry mechanism positioned in an exhaust flow path to a turbine wheel. The terms variable nozzle turbine (VNT) and variable geometry turbine (VGT) are often used to refer to such a mechanism. A VNT typically includes pivotable vanes where each vane has a pair of surfaces extending from a leading edge to a trailing edge where a surface of one vane and a surface of an adjacent vane form a nozzle. As the vanes pivot, the nozzles change shape. In other words, as the vanes pivot, the geometry of the nozzles varies.
A VNT operates in a harsh and changing environment. Exhaust temperatures cycle with operating conditions (e.g., acceleration, deceleration, demand, etc.) and may exceed 500° C. A variable geometry mechanism should be robust and capable of handling environmental and operational conditions. A robust mechanism should be capable of controlling geometry precisely; in a manner that does not vary significantly from unit to unit. Various exemplary methods, devices, systems, etc., described herein pertain to variable geometry mechanisms with improved control and durability.